Deep brain stimulation (DBS) and other related procedures involving the implantation of leads and catheters in the brain are increasingly used to treat such conditions as Parkinson's disease, dystonia, essential tremor, seizure disorders, obesity, depression, motor control disorders, and other debilitating diseases. During these procedures, a catheter, lead, or other medical device is strategically placed at a target site in the brain. Locating the “best” or optimal site in the brain for deep brain stimulation can be a painstaking procedure.
Implantation of a lead for DBS generally involves the following preliminary steps: (a) anatomical mapping and (b) physiological mapping. Anatomical mapping involves mapping segments of an individual's brain anatomy using non-invasive imaging techniques, such as magnetic resonance imaging (MRI) and computed axial tomography (CAT) scans. Physiological mapping involves locating the brain site to be stimulated. Step (b) can be further divided into: (i) preliminarily identifying a promising brain site by recording individual cell activity with a microelectrode and (ii) confirming physiological stimulation efficacy of that site by performing a test stimulation with a macroelectrode or microelectrode.
Microelectrode recording is generally performed with a microelectrode recording (MER) system. The MER system includes a small diameter electrode with a relatively small surface area optimal for recording single cell activity. The microelectrode may essentially be an insulated wire that has at least the distal portion uninsulated to receive electrical signals. The microelectrode functions as a probe to locate an optimal site in the brain for deep brain stimulation. Activity detected through the microelectrode is recorded by the MER system. Since a number of attempts may be required to locate the optimal site, it is desirable that the microelectrode be as small as possible to minimize trauma when the microelectrode is introduced into the brain, in some cases, multiple times.
Once an optimal site in the brain for deep brain stimulation has been identified by the microelectrode recording, a macroelectrode is used to test whether the applied stimulation has the intended therapeutic effect. Once macrostimulation confirms that stimulation at the optimal site provides the intended therapeutic effect, the macroelectrode is withdrawn from the brain and a DBS lead is permanently implanted at the optimal site in the brain for deep brain stimulation.
There are a number of commercially available MER systems used in deep brain stimulation. One exemplary MER system permits the neurosurgeon to simultaneously record an output from five different microelectrodes, referred to as “five-at-a-time.” In this approach, five microelectrodes are advanced into the brain at the same time and at the same speed. This presents obvious advantages. The set-up time may be proportionately cut, since the chance of locating an optimal stimulation site theoretically increases by five fold. However, the size and configuration of this system is more likely to cause damage to brain tissue. For instance, because the microelectrodes in a “five-at-a-time” system are placed relatively close to each other, two of these electrodes may sometimes “capture” a blood vessel between them. This may result in vessel punctures and may lead to intracranial bleeding. In contrast, when a single microelectrode is used, a blood vessel can often escape injury because the vessel can deflect away from the microelectrode, or vice-versa. Thus, some neurosurgeons choose to use an MER system with only a single microdrive, advancing one microelectrode at a time until an optimal stimulation site is found.
The recorded output of a microelectrode advanced along a path through the brain is referred to as a recording tract. Some neurosurgeons average four to five microelectrode recording tracts before they decide on an optimal site in the brain for deep brain stimulation. Other neurosurgeons only use one recording tract, which cuts surgery duration, but which may not locate the optimal stimulation site. Without optimal electrode placement, the DBS lead may need to be driven at a higher current to produce the desired therapeutic effect. This, however, can cause the device battery to be drained more quickly. In addition, the use of higher currents can increase the risk of undesirable side effects such as dysarthria (slurred speech) and abulia (an abnormal inability to make decisions or to act).
Each of these MER systems applies the conventional surgical procedure of creating multiple microelectrode tracts until an optimal site for deep brain stimulation is found within the brain. On average, a single microelectrode recording tract takes approximately thirty minutes to perform. Each microelectrode recording tract requires placement of the microelectrode via a larger diameter insertion cannula through viable brain tissue. Each time an object is inserted into the brain there is approximately a five percent risk of hemorrhage. Creating multiple tracts increases the risk for intracranial bleeding, duration of operation, post-operative infection, and operative risk. Creating new tracts is fraught with misalignment/misplacement problems because the introduction cannulas may not always trace the exact pathways desired.